Laser frequency locking is an essential technology for optical communications and other fields. In telecommunications, Dense Wavelength Division Multiplexing (DWDM) systems require tight control and accurate tuning of each frequency propagated down an optical fiber by a communication laser. In DWDM, each of a plurality of laser signal sources is tuned in frequency to a distinct channel, allowing a plurality of signals to be simultaneously transmitted down a single optical fiber. In this way, large volumes of information can be transmitted through a single optical fiber. The communication channels are defined on a grid with equal frequency spacing in a band range approximately between 188-197 THz (the ITU grids).
Each laser must be stabilized, or “locked” to ensure it remains frequency-tuned to the proper communications channel. Frequency locking techniques must withstand environmental or systematic disturbances. A wavelength locker provides a stable and calibrated reference for measuring the wavelength deviation of a laser output from a desired wavelength. The signal from the wavelength locker is used to tune the laser wavelength back to the desired frequency. In a DWDM system, mistuning is highly detrimental to the performance of communications since DWDM components exhibit wavelength-dependent losses. In other systems, mistuning a laser can have various negative ramifications depending on the measurement, analysis or process facilitated by the laser. In telecommunications applications, wavelength lockers are critical because they allow for more closely spaced channels, thus increasing the information bandwidth of a DWDM system.
It is common to use a Fabry Perot etalon as a reference element in a wavelength locker. A Fabry Perot etalon is an interferometric device composed of 1 mm solid silica with partially-reflecting mirrored sides that are substantially parallel. A light beam passing through an etalon is expressed as an Airy function and the separation in frequency between the period peaks of the transmission response is called the Free Spectral Range (“FSR”). The FSR is defined by the optical path length of the gap between the etalon mirrors. Typically, a wavelength locker based on a Fabry Perot etalon will be designed so that the FSR of the etalon is matched against the frequency spacing of the ITU grid so that the etalon provides a calibrated reference to indicated the frequency location of the ITU channels.
Typically, frequency locking is performed in a feedback loop whereby the output of the laser is tapped and coupled to the wavelength locker which tracks the output from its etalon with a photodetector. The side of the etalon transmission peak is monitored for frequency discrimination against a reference value. The difference between the monitored transmission and the reference value is proportional to the deviation of the lasing frequency from the desired lock point. The difference then used as feedback into the laser control electronics, to adjust the lasing frequency to the desired lock point.
However, one known issue with the use of Fabry Perot etalon as a reference element in a wavelength locker is that power fluctuations in the input light are also capable of producing changes in the etalon transmission signal that mimic a frequency change, resulting in the potential to unintentionally detune the laser from the desired lock point. It is common to use a second photodetector to tap a power reference signal from the output of the laser to ensure that power fluctuations are differentiated between a frequency change. The power reference signal is used to normalize the etalon transmission signal to render it insensitive to changes in the input optical power.
Another known issue with the use of Fabry Perot etalon as a reference element in a wavelength locker is that temperature fluctuations to the etalon changes the optical path length of the etalon due to the material's thermal coefficient of expansion, thereby changing the FSR and peak locations of the Fabry Perot etalon and causing the laser to detune from the desired lock frequency. Thermally-induced changes to the etalon are normally mitigated by constructing the etalon from temperature-insensitive materials and mixing material stacks with offsetting coefficient thermal expansion (CTE) properties. However, the materials generally used to construct the etalon also exhibit a temperature-dependent refractive index. Often the only way to mitigate temperature fluctuations from impacting the performance of the device is to assemble and mount the etalon on a temperature controlled platform. The long lifetimes of telecommunications systems demand that wavelength lockers operate robustly over a very long period exceeding 20 years. Wavelength lockers most also be constructed to avoid the outgassing of superfluous material, to survive mechanical and thermal shock, and otherwise not age in an observable or detrimental way. Epoxies and adhesives used to assemble wavelocker systems are notorious for age-dependent power losses, outgassing, and inadvertent etalon effects.
In addition to the technical requirements of a wavelength locker device, the telecommunications market demands that wavelength lockers are physically no greater than 33 mm3 and small enough to fit inside a 14-pin butterfly package along with the packaged laser. In addition, the telecommunications market demands that wavelockers exhibit a long lifetime and are inexpensive.
Finally, existing wavelockers are generally fixed and therefore require active alignment to match the 50% point exactly. Also, since each filter is optimized for a single wavelength, laser manufacturers are forced to stock an abundance of parts each having a frequency that matches the wavelength of the various lasers the offer.
As so, there is a need for a wavelength locker that has a small physical size. There is also a need for a wavelength locker with a size suitable for placement inside an associated laser package. There is a further need for a wavelength locker that operates robustly over a very long lifetime. Yet another need for a wavelength locker that does not use epoxies and adhesives in the beam path.